Development of a wearable artificial kidney is envisioned, based on a new concept in blood purification: direct contact of blood with a miscible rinsing fluid. A system that uses this concept would require several components. This proposal is sharply focused on the critical component, a membraneless contactor in which a thin sheet of blood is sheathed by two sheets of rinsing fluid. The three sheets flow together for about 1 sec without mixing and are then separated. Nanopore filters are used to assure that the separated rinsing fluid contains no cells. The speed of blood toxin transfer to the rinsing fluid is much higher than in current devices and the area of contact is much less. Blood contacts only another fluid, no solid, in the contactor. The proposed research would prove that mixing is limited, that the flow is stable, that transfer is as rapid as calculated, and that the filters retain cells without clogging. A clinically effective contactor would be very small and easily worn. If successful, this system would be the first major change in dialysis therapy in 35 years, and would be the first application of microfluidics and nanofiltration to therapeutic medicine. This system would allow steady, more physiological compensation for kidney failure than is possible with current intense episodes of treatment in clinics. It would be one of the first innovations in dialysis therapy with the potential of lowering the cost of therapy. However, even if the membraneless contactor is fully realized, other technical problems, thought to be less difficult but still significant, must be solved before the device could be made available to the general patient population.Early development of a wearable artificial kidney for the treatment of end-stage renal disease (ESRD) is envisioned. It is based on a new concept in blood purification: direct membraneless contact, without mixing, of blood with a miscible rinsing fluid.